7.13: Diamagnetic Shielding of Nuclei: Local Diamagnetic Current
An applied magnetic field causes the electrons present in the molecule to circulate, setting up a local diamagnetic current within the molecule. The local diamagnetic current arising from circulating sigma-bonding electrons induces a magnetic field, Blocal that opposes the applied magnetic field, B0. The effective magnetic field experienced by these nuclei is given by the difference between the applied and local magnetic fields in a phenomenon called local diamagnetic shielding. Essentially, Blocal increases with the electron density surrounding the nuclei, leading to increased shielding and a lower Beffective. Since electron densities vary within a molecule, nuclei in the same molecule are shielded to different extents and experience different effective fields. A nucleus in an electron-dense environment is well-shielded from the applied magnetic field and experiences a lower Beffective. Consequently, the energy required to flip its spin is less than that required for a poorly shielded nucleus in electron-poor surroundings. Thus, shielded nuclei experience resonance at lower frequencies than deshielded nuclei. Resonance frequencies are plotted on the NMR spectrum, making these spectra sensitive to diamagnetic shielding.
